This invention relates to a coaxial cable transition to a planar substrate arrangement, such as a coaxial cable to microstrip arrangement.
Coaxial cable is widely employed in system configuration, where microwave and radio signals are processed. A typical use of a coaxial to planar substrate transition is in a mobile communications network base station where receive and transmit electronics are connected to a triplate or layered antenna by way of a coaxial cable. One form of triplate antenna comprises a microstrip feed network printed on a dielectric film or substrate which provides the feed probes or patches which extend into or are arranged within radiating apertures defined through the outermost groundplane of the triplate antenna. In such an arrangement, the central conductor of a coaxial cable is soldered directly to the microstrip circuit of the antenna. The axis of the central conductor can either be in-line or orthogonal with respect to the substrate and the earthed sheath is connected to the groundplanes of the antenna. Alternatively, the microstrip array may be formed upon a printed circuit board manufactured from a substance such as PTFE. U.S. Pat. No. 4,918,458 (Ford Aerospace) describes such an antenna arrangement which is fed by way of a coaxial supply cable. GB-A-2007919 (Raytheon) also provides an antenna arrangement which is fed by way of a coaxial supply cable.
These types of configuration, while easy to manufacture can suffer from the generation of passive intermodulation products. Power handling capabilities can be limited since high losses will result from the isolating distances necessary from the coaxial transition section to any power dividers such as Wilkinson couplers. Further problems arise in the use of the dielectrics having high temperature capabilities necessary in order to allow solder connections to be made. Coupled lines can be present in order to provide a d.c. block in cases such as active antennas.
In the design of mechanical connections with microwave conductors, extreme care needs to be exercised for critical applications requiring high linearity, for example, cellular radiocommunications and satellite communications. In the case where components are welded or soldered, attention needs to be paid to the electrical conductor's surface; irregularities and imperfect metal to metal contacts lead to electrical non-linearities. This introduces passive intermodulation, in which deleterious, spurious signals are generated and, generally, these effects vary with frequency, contact pressure, age and other factors.
An object of the invention is to provide an improved coaxial cable to microstripline connection with high mean or peak power handling and very low passive intermodulation product generation.